Question: Which of these numbers is prime? ${17,\ 30,\ 65,\ 85,\ 99}$
A prime number has exactly two factors: 1 and itself. The factors of 17 are 1 and 17. The factors of 30 are 1, 2, 3, 5, 6, 10, 15, and 30. The factors of 65 are 1, 5, 13, and 65. The factors of 85 are 1, 5, 17, and 85. The factors of 99 are 1, 3, 9, 11, 33, and 99. Thus, 17 is a prime number.